Corneal keratectomy is a surgical procedure which changes the refractive characteristics of the cornea by making deep incisions on the cornea. The procedure is used to eliminate or reduce eye disorders such as myopia, hyperopia and astigmatism.
Incision depth is the most critical factor in determining the refractive outcome to the keratectomy surgery. Results of the conventional technique are found to be inconsistent due to the use of steel or diamond knives. The corneal incisions have been found to vary considerably from the expected depth, with ranges from 66% to 96% for steel and 61% to 98% for diamond knife incisions.
It was discovered in or about 1981, that the corneal epithelium exhibits an unusual sensitivity to 193 nm laser light. Laser light of this wavelength was shown to be capable of etching organic substrates in or about 1982. It was then shown that this etching effect would similarly occur with biological tissue. More particularly, in 1983, in a series of experiments, (Trokel, S., Srinivasan, R. and Branen, B., Am. J. Opthal. 96:710 (1983)) the corneal stroma was ablated with an accuracy comparable to organic substrates.
The excimer laser is the most convenient source of UV laser light and has 193, 249, 308 and 351 nm light as major emission lines. These wavelengths span the UV spectrum and provide a suitable range with which to examine corneal tissue interactions. These four wavelengths can be produced at sufficiently high irradiance to cause corneal tissue ablation.
Therefore, the technique of UV laser keratectomy, was developed to replace conventional surgical corneal sculpting using diamond knives. The use of excimer lasers for corneal sculpting has proliferated during recent years.
The sharply controlled cutting/ablation ability of excimer lasers contrasts with the less controlled techniques which use the longer wavelength infrared laser frequencies which are also absorbed by corneal tissue. A CO.sub.2 laser, emitting light at 10.6 .mu.m, produces an incision with irregular jagged edges, which chars the collagenous material of stroma (Keates et al 1981). This effect is quite unlike the uniform incisions produces by the 193 nm emission of the excimer laser. Consequently the excimer laser has developed into a preferred apparatus for corneal surgery.
Excimer lasers have been used to perform three types of refractive surgery. The first type uses the laser to create linear or circular excisions of corneal tissue, similar to conventional incision techniques. These incisions are effected to alter the mechanical stress patterns of the incised corneal stroma. In the untouched central cornea a new stress equilibrium is established, which produces a new optical curvature. The second type of refractive surgery involves ablation of surface areas of the cornea to create a new anterior curvature variously known as surface etching, reprofiling, surface ablation, and laser keratomileusis. The third type of refractive surgery involves removing tissue from discs of excised cornea in keratomileusis and epikeratoplasty.
Among numerous parameters that must be defined and controlled for these operations are the uniformity of the laser beam, the amount of radiant exposure, the energy distribution achieved on the cornea, the repetition rate of the laser, the total number of pulses, the total amount of energy delivered, as well as the shape and energy distribution of the laser beam as it impacts the cornea.
The underlying assumption for UV laser keratectomy is that if all of the individual parameters are set satisfactorily the overall result will be accurate. There are over ten parameters relating to this procedure. The above assumption ignores the interdependencies of these parameters. Currently there is no means of checking the overall result of these interdependencies on the actual cornea. For example, changing the intensity distribution of the beam will affect the energy per pulse and the resulting focusing characteristics. This in turn will affect the fluence on the cornea and ablation characteristics. If several of the parameters are readjusted prior to the operation then the overall result on the cornea becomes unpredictable.
The only way to ensure the repeatability of this surgical procedure is by characterizing the overall effect of the procedure on the cornea. A number of different lasers and surgical systems, each with a different number of parameters and optical path characteristics are currently being used. It is impossible to compare the results achieved with these intrinsically different systems.
It is also a well documented fact that excessive UV exposure to the eye causes and contributes to, among other effects, cataracts, uveal melanoma and photoreceptor insensitivity. Therefore, it is very important to determine and record the amount of UV radiation received by the eye during the operation. Currently, this in not being done during UV laser corneal sculpting surgery. Success of the operation is always judged after the operation.